[dm-devel] Re: Snapshot merging

[Mike Snitzer <snitzer redhat com>]

On Wed, Sep 09 2009 at 11:56am -0400,
Mike Snitzer <snitzer redhat com> wrote:

> On Wed, Sep 09 2009 at 11:47am -0400,
> Mikulas Patocka <mpatocka redhat com> wrote:
> 
> > Hi
> > 
> > I found one bug in the userspace code --- start merging, then from the 
> > other console deactivate the origin volume with lvchange -an, then wait 
> > next 15 seconds for the next poll interval:
> > 
> > the poll process will think that the merging finished, removing the 
> > snapshot, leaving partially merged origin and causing data corruption:
> > 
> > [slunicko:~]# lvconvert -M vg1/lv2_snap1
> >   Merging of volume lv2_snap1 started.
> >   lv2: Merged: 13.1%
> > ( .... deactivate the volume from the other console ....)
> >   lv2: Merged: -1.0%
> >   Merge into logical volume lv2 finished.
> >   Logical volume "snapshot4" successfully removed
> > 
> > Maybe this bug was in my code too, I don't remember.
> 
> Likely, considering I merely ported your LVM2 patches to the latest
> version.  That said, I could've introduced this issue by missing
> something in the port.  Regardless, I'll have a look at preventing the
> origin LV from being deactivated during a merge.

Or more correctly: gracefully halt the merge and deactivate merging
snapshot along with the origin.  Then on activation the merge should
just resume (in the background).

Mike